It is known that flat film type oxygen sensors which use a flat solid electrolyte layer show better performance in EMF (electromotive force) characteristic and responsiveness as compared with tubular type oxygen sensors which use a tubular solid electrolyte. However, some of such oxygen sensors fail to exhibit their maximum performance because of inhomogeneous distribution of a stabilizer in the body of the solid electrolyte, originating from the inherent production method of the sensors. This drawback encountered in the conventional oxygen sensors will become apparent hereinafter.